Combined cycle power plants have come into widespread use because they incorporate heat exchangers that can recover heat from the hot gas exhaust stream of a combustion engine. Conventionally the recovered heat is used to generate the working fluid of a steam turbine. This results in more efficient power generation than is achievable with only a combustion turbine or only a steam turbine. See, for example, U.S. Pat. No. 5,375,410 which is assigned to the assignee of the present invention and incorporated herein by reference.
Generally, combined cycle power generation systems include a first power source which operates on a Rankine cycle, e.g., a steam cycle, and a second power source based on a combustion process wherein heat recovered from the hot exhaust gases of the combustion process is transferred to the working fluid in the Rankine cycle. Such systems render overall plant efficiencies on the order of 55 percent or higher. System operations are subject to numerous constraints in order to reduce undesirable atmospheric emissions and to reduce deleterious effects on the complex, high speed mechanical components. It is therefore necessary to accurately monitor and control numerous processes in power generation systems.
For example, the prevalence of strong and weak acids in a conventional steam cycle varies with time, and the ability to accurately monitor constituents, such as hydrochloric acid or acetic acid, is critical to controlling these chemicals to levels which maximize the useful life of the mechanical components. However, when monitoring strong acid content based on cation conductivity, weak acid content, e.g., resulting from entrainment of low levels of carbon dioxide in the steam cycle, can mask the presence of the strong acids. This problem occurs at carbon dioxide concentrations on the order of less than 0.03 percent based on atmospheric intrusions. For example, the steam cycle condenser is normally maintained under a partial vacuum, but when the system is shut down the vacuum is lost. Consequently, small amounts of carbon dioxide can enter the system and be absorbed into ammonia-rich feedwater. When the system starts up, such weak acid content can accompany the working fluid through the cycle.  One method of preventing such carbon dioxide intrusion would be to maintain the vacuum while the steam turbine is shut down, and provide a continued supply of steam to the associated gland seals. A method for removing carbon dioxide from the working fluid is to simply vent the fluid, but this can require exhausting relatively large amounts of the hot fluid, resulting in undesirable thermal losses. The associated expense of either solution is to be avoided.
Combustion process constituents in combined cycle power generation systems also must be accurately monitored and controlled, e.g., for environmental reasons. The control process can be complex because the levels of chemical emissions vary as a function of operating state. Ideally, power generation processes might be simpler to control under steady state conditions, but dynamic and varied power output is often necessary or desirable. For example, during starting-up combustion turbines can emit undesirably high levels of regulated emissions, especially when the power output levels are less than seventy five percent of the maximum steady state power output. To reduce the NOx emissions (e.g., NO2 and NO3) during start-up of the combustion process, it is necessary to limit flame (combustion) temperature.
Generally it is desirable to find more effective and efficient ways to reduce the adverse effects of constituents present in power generation processes.